1. Field of the Invention
This invention relates to a laser beam machining device.
2. Description of the Prior Art
FIG. 10 shows a conventional X-Y table type laser beam machining device, in which a laser beam 2 emitted from a laser beam oscillator 1 is deflected 90 degrees and irradiated on a workpiece 9 by the converging lenses 11b. The workpiece 9 is moved by the X-Y table 10 and machined in a prescribed way.
FIG. 11 shows a conventional galvanometer type beam scanning device which is a laser beam machining device, in which a laser beam 2 emitted from a laser oscillator 1 is irradiated on a workpiece 9 by an f.theta. lens 17 while scanning a plane by means of two galvanomirrors 16 moved by a galvanometer 15.
However, the above-mentioned method that uses one laser beam so as to form a machining mark requires much time and is not productive when the same machining must be repeated on the workpiece 9. Therefore, laser beam machining devices such as that shown in FIG. 12 are often used, which are capable of machining at multiple points on a workpiece. The laser beam machining device of FIG. 12 has convex lenses 3b and 4b for enlarging the beam diameter of a laser beam 2a, and convex cylindrical lenses 5 and 6 for shaping the cross section shape of the laser beam 2d, and a converging optical unit 8 with planoconvex lenses 11. The laser beam 2a emitted from the laser oscillator 1 passes through the convex lens 3b, by which the laser beam 2a is converged and then diverged. Then, it passes through the convex lens 4b with a longer focal length than the convex lens 3b, resulting in a laser beam 2d with a uniform beam diameter larger than that of the laser beam 2a. The laser beam 2d is converged and then diverged horizontally by the convex cylindrical lens 5 and then passes through the convex cylindrical lens 6 which has a longer focal length than the convex cylindrical lens 5, resulting in a parallel laser beam 2e that is diverged horizontally wider than the laser beam 2d. The laser beam 2e is directed to the converging optical unit 8 by a reflecting mirror 7, and by each of the planoconvex lenses 11 of the converging optical unit 8, the laser beam 2e is then converged onto the workpiece 9, resulting in multiple spots on the workpiece 9. The workpiece 9 is moved by the X-Y table 10 to perform a prescribed machining.
Generally, the cross section intensity distribution of a laser beam emitted by solid state lasers (e.g., Nd-YAG lasers, etc.) or gas lasers (e.g., C02 lasers, etc.) is not uniform and demonstrates a Gaussian distribution such as that of FIG. 13a. Moreover, the cross section intensity distribution of the laser beam 2e still demonstrates a Gaussian distribution even after the beam diameter of the laser beam 2a is diverged or the shape of the beam's cross section is changed in other ways. That is, as shown in FIG. 13a, the intensity of the laser beam 2e is highest at its center and decreases exponentially toward its edges.
Moreover, when a laser beam 2e with this kind of Gaussian distribution is converged by the converging optical unit 8 that is composed of planoconvex lenses 11 and irradiated on the workpiece 9 into multiple spots, the laser energy density (laser beam intensity per unit area) at each of the machining spots 12 also demonstrates a Gaussian distribution because it is the highest in the middle of the laser beam and lower toward the edges of the laser beam.
FIG. 13b shows the relationship between the machining condition range and the allowable machining area with respect to the laser beam intensity, in which the hatched area indicates the energy levels of the laser beam 2e that can be used effectively in machining. The remaining areas indicate the energy levels that cannot be used effectively in machining. Since the cross section intensity distribution of the laser beam 2e is not uniform and has a Gaussian distribution, the allowable machining area is limited by the machining condition range of the laser beam intensity, so all of the energy of the laser beam 2e cannot be used effectively in machining. For example, consider a machining case wherein a laser beam continuously emitted from a YAG laser is pulsed by a Q switch and irradiated on a vapor deposition film with a 1000-.ANG.-thick Al layer in such a way that only the upper Al layer of the film is removed without damaging the film. By performing this machining using the laser beam machining device shown in FIG. 12, the film was damaged when the laser energy density exceeds 2.times.10.sup. 3 J/cm.sup.2, and the Al layer on the deposition film was not removed when it dropped below 1.6.times.10.sup.3 J/cm.sup.2. That is, the machining condition range for the laser energy density is extremely narrow, 1.6.times.10.sup.3 -2.times.10.sup.3 J/cm.sup.2, so that the allowable machining area becomes small and only about 30 percent of the total energy of the laser beam can be effectively used.